We present a model for representing competing classifications in biological databases. A key feature of our model is its ability to support future classifications in addition to current and previous classifications without reorganizing the database. Data in biological databases is typically organized around a taxonomic framework. Biological data must be interpreted in the context of the taxonomy under which it was collected and published. Since taxonomic opinion changes frequently, it is necessary to support multiple taxonomic classifications. This is a requirement for providing comprehensive responses to queries in databases that contain data reflecting incompatible taxonomic classifications.